Middle relief is the most unheralded position in baseball. In real life, middle relievers are usually the weakest links on their team’s pitching staff and in fantasy baseball they are usually ignored altogether.
In roto leagues you would be very wise to draft one or two middle relievers on your team, especially in deeper mixed leagues and in NL or AL-only leagues. Keep in mind that in head to head points leagues middle relievers are pretty useless. They are not as valuable in head to head roto leagues as well, unless your league allows daily lineup changes.
Some middle relievers can help out your ratios pretty significantly while others can give you some rogue saves.
There is constantly closer turnover in the MLB. Guys like John Axford and Neftali Feliz started 2010 as setup guys, but they were closing games before too long. Roto teams are always chasing saves so why not get ahead of the curve and draft a couple middle relievers that have a chance at getting you some saves.